Human exposure to aerosols from indoor, outdoor, or workplace causes adverse health effects. The United States Environmental Protection Agency (EPA) promulgates regulations on PM10 (mass of particles with aerodynamic diameters less than approximately 10 μm) and PM2.5 (mass of particles with aerodynamic diameters less than approximately 2.5 μm). The American Conference of Governmental Industrial Hygienists (ACGIH) has also established regulations on respirable, thoracic and inhalable aerosols, defined as particles having aerodynamic diameters of less than 4 μm, 10 μm, and 100 μm respectively. A discussion of the various regulations are found at National Primary and Secondary Ambient Air Quality Standards, 40 Code of US Federal Regulation, Chapter 1, Part 50 (1997) and Vincent, J. H., Particle Size-Selective Sampling for Particulate Air Contaminants Cincinnati, ACGIH (1999), both of which are hereby incorporated by reference except for explicit definitions contained therein.
Presently, the federal reference method (FRM), which utilizes filter samplers, is implemented to determine compliance with mass based air quality standards. The particle size collected by the filter samplers is determined by a size selective inlet. Typically, the filter method requires a relatively long sampling time, about 24 hours, to collect enough mass on the filter, with the results not being available until the samples are analyzed in the laboratory.
Direct-reading instruments provide near real-time measurement of aerosol mass concentrations. For example, a photometer can be calibrated to surrogate fine particle mass concentration over a wide concentration range. But it does not provide size information. On the other hand, an optical particle counter (OPC) or aerodynamic particle sizer (APS) may provide very high resolution size distributions. But these instruments only work at relatively low concentrations due to coincidence errors.
U.S. Patent Publication Number US 20090039249A1 (U.S. patent application Ser. No. 12/187,287) commonly assigned to the assignee of the present invention, and to which the present application claims priority, discloses an invention that combines photometry and optical or aerodynamic particle sizing in one optical device for measuring size segregated mass concentrations, for example, PM10, PM2.5 and PM1, or inhalable, thoracic and respirable fractions, in real time. The optical device features a single optical chamber and a single detector. Such a design reduces instrument components and simplifies the instrument configurations. However, such a design may pose challenges to measuring very low concentrations using photometric signals.
Optical or aerodynamic sizing requires highly focused light beam(s) to produce a strong scattering pulse. To achieve a highly focused light beam, optics having shorter focal lengths are typically employed. Such optics have wide convergence/divergence angles, thus bathing larger surface areas. For example, optics and light traps can create a large amount of stray light, and cause great difficulties to design apparatuses, for example apertures, in reducing stray light. Because the photometric signal is detected by the same detector in the same optics chamber, the stray light causes low signal-to-noise levels of the photometric measurement at very low concentrations, for example, at concentrations in the range of 0.1-1 μg/m3. The stray light also causes background photometric signal drift at different environment temperatures.